The present invention relates to the active layer in an electroluminescent device.
A typical electroluminescent device is a multilayer thin film structure that emits visible light when activated by an applied voltage. The active layer in such a device will contains a phosphor. This active layer has previously been deposited applying various curable compositions to a suitable substrate followed by ultraviolet (UV) light curing or heat curing. The usual compositions, however, contain organic solvents that do not incorporate into the active layer after curing. Such solvent based systems are undesirable because of the hazards and expenses associated with volatile organic solvents.
UV radiation curable compositions are applied to a substrate through spraying, screen printing, dipping or brushing for the protection or decoration of the substrate. In the usual application, a substrate such as metals, glass, or plastics is coated with the composition and then UV light is introduced to compete the curing process. The UV curable compositions offer many advantages over typical heat curable compositions.
Heat curable compositions require the use of organic solvents that contain a significant amount of volatile organic compounds (VOCs). These VOCs escape into the atmosphere while the heat curable composition dries. Such solvent based systems are undesirable because of the hazards and expenses associated with VOCs. The hazards include water and air pollution and the expenses include the cost of complying with strict government regulation on solvent emission levels. In contrast, UV curable compositions contain reactive monomers instead of solvents; thus eliminating the detrimental effects of the VOCS.
The use of heat curable compositions not only raises environmental concerns but other disadvantages exist with their use as well. Heat curable compositions suffer from slow cure times which lead to decreased productivity. These compositions require high energy for curing due to energy loss as well as the energy required to heat the substrate. Additionally, many heat curable compositions yield poor film properties that result in decreased value of the end product.
In a typical electroluminescent device, the active layer comprises one layer of a multilayer electroluminescent device. An example of such a device would contain a substrate made of polycarbonate or glass coated with a transparent conductor such as fluorine doped tin oxide. Metallic grid lines are patterned onto the substrate. The active layer is then applied by screen printing the electroluminescent composition onto the substrate with gridlines. A dielectric coating is then optionally applied over the structure. Finally, the device is coated with a metallic backing. The active layer is such electroluminescent devices typically contains a phosphor. Such phosphor may or may not be encapsulated with various oxides or nitrides. Encapsulation protects the phosphor from the deleterious environmental effects.
It is an object of the present invention to provide an improved electroluminescent composition that is curable by ultraviolet light.
It is another object of the present invention to provide an improved electroluminescent composition that can be applied by spraying, screen printing, dipping, and brushing.
It is still another object of the present invention to provide an improved electroluminescent composition that comprises either an encapsulated or an unencapsulated phosphor.
It is yet another object of the present invention to provide an improved electroluminescent composition that comprises at least one aliphatic acrylated oligomer that can be used to coat a substrate such that no significant amount of volatile organic solvents do not become incorporated in the coating after the composition is cured.
The present invention discloses an ultraviolet light curable electroluminescent composition and method for making such a composition that may be used to produce an electroluminescent active layer. In this context, an active layer is a layer that when incorporated in a suitable device emits light when a voltage is applied. The disclosed composition does not contain any significant amount of volatile organic solvents that do not become incorporated in the active layer after curing. Specifically, the electroluminescent composition contains 5% or less volatile organic solvents by weight. It is an advantage of the present invention that the deposition of the dielectric layer in such a device is optional when the disclosed electroluminescent composition is used to deposit the active layer.
In accordance with one aspect of the invention, an ultraviolet light curable electroluminescent composition is provided. The electroluminescent composition comprises a mixture of one or more aliphatic acrylated oligomers, wherein the aliphatic acrylated oligomer mixture is present in an amount of about 10% to 40% of the electroluminescent composition. All percentages of the electroluminescent composition as expressed in this document refer to the weight percentage of the stated component to the total mass of the electroluminescent composition.
The electroluminescent composition preferably comprises an isobornyl acrylate monomer in an amount of about 4% to 30% of the electroluminescent composition, optionally an adhesion promoter in an amount of 1% to 10%, a photoinitiator in an amount of about 0.5% to 6% of the electroluminescent composition, optionally, a flow promoting agent in an amount of about 0.1% to 5% of the electroluminescent composition, and an electroluminescent phosphor in an amount of 28% to 80%. The electroluminescent phosphor may either be encapsulated or unencapsulated.
In accordance with yet another aspect of the invention, a method is provided for depositing a electroluminescent coating on a substrate. The method comprises a first step of applying to the substrate a electroluminescent-containing fluid-phase composition (xe2x80x9celectroluminescent compositionxe2x80x9d). The electroluminescent composition comprises a mixture of aliphatic acrylated oligomers, wherein the aliphatic acrylated oligomer is present in an amount of about 10% to 40% of the electroluminescent composition. The electroluminescent composition also includes an isobornyl acrylate monomer in an amount of about 4% to 30% of the electroluminescent composition, a photoinitiator in an amount of about 0.5% to 6% of the electroluminescent composition, a flow promoting agent in an amount of about 0.1% to 5% of the electroluminescent composition, a copper activated zinc sulfide electroluminescent phosphor in an amount of 28% to 80%, and optionally an adhesion promoter in an amount of 1% to 10%.
The method also includes a second step of illuminating the electroluminescent composition on the substrate with an ultraviolet light to cause the electroluminescent composition to cure into the electroluminescent coating.
In accordance with this method, the electroluminescent composition can be selectively deposited on the substrate at specific locations where electroluminescent plating is desired. It need not be applied to the entire substrate.